content@managingwater.co.uk Managing & Maintaining our water channels BIO-ENGINEERING


OF WATERWAY BANKS BRINGS TANGIBLE EROSION CONTROL BENEFITS


DAVID HOLLAND, SALIX


“traditional hard engineering options have a detrimental impact on habitat quality which will reduce the


likelihood that a watercourse will meet good ecological status”


David Holland, Technical Director of Salix RW, the UK’s leading designer and supplier of bio-engineered sustainable solutions to soil and riverbank erosion, discusses the role of geotechnical materials in advanced ‘soft’ erosion control techniques for protecting watercourses and channels. While highlighting the benefits of using materials with known performance, he warns of the drawbacks of poor understanding of the fundamentals of design and construction and looks at projects where the right approach offered enhanced value beyond cost savings.


The consequences of the requirements of the EU’s Water Framework Directive (WFD) have had many benefits. Not least of these has been where hydrological and civil engineers have started to make the connection between flood protection defences and flood prevention through SUDS based methods of surface water volume and quality control. Water quality is one aspect that contributes to the target of good ecological status along with physical habitat structure


“One of the problems has been that engineers have traditionally viewed protection and control in terms of solid structures, with calculations based on mass and strength to withstand the pressure and force of flowing water,” observes David Holland.


“Concrete revetments and facings, stone riprap, gabions and similar solutions are often the first methods considered. It’s perhaps based on traditional coastal protection measures, such as those practised by Victorian engineers in building harbours, but the hard design attitude is still very pervasive. These traditional hard engineering options have a detrimental impact on habitat quality which will reduce the likelihood that a watercourse will meet good ecological status”


Directives to reduce water pollution


“low carbon or even carbon neutral


approach”


However, to meet the WFD’s objective of achieving good ecological status water course environments from river sources to their local coastal marine environments, engineers must look at water quality control measures and ecological habitats


These include reducing the excess load of sediment and silt carried in rivers and channels, which can be achieved by reducing erosion as a direct or indirect result of human activity. Sediments and silts are also proven carriers of pollution, adsorbed onto the surface of particles carried in flow; contaminants can include hydrocarbons (e.g. from roads), toxic metals and undesirable organic materials.


26 Recreating natural control


Under natural conditions, waterway banks are vegetated with a range of plants which find the growing conditions very favourable. As a consequence, as well as preventing erosion and stabilising the soil, this vegetation also provides excellent traps for silts and sediments being carried by surface water flow over the surrounding land and across the banks to the water course.


The increasing pace of climate change and its accompanying incidence of storm events have thrown the problems of water- related erosion into sharp relief. Areas of the country with spate river systems characterised by sharp rises in level are particularly vulnerable, such as Wales and Northern England.


SUDS focus benefits


More attention is being paid to providing better protection from surface water by channelling flow, as well as refurbishing the existing networks of local drainage which were allowed to fall into disrepair in the late 20th century. With the growth of appreciation of the role of Sustainable Urban Drainage Systems (SUDS), the benefits of natural measures such as vegetated swales and ponds in mitigating flow and poor quality of surface water is also better understood.


For example, the Scottish Environment Protection Agency (SEPA), through its Pollution Prevention Guidelines, has formalised the incorporation of measures such as vegetated filter strips in providing run off control, for example alongside highways, especially those adjacent to rivers. As part of scheme proposals, designers have to demonstrate to planners that two or three different treatment measures are incorporated to protect water courses.


“The benefits are not just aesthetic”, states David, “although this is often the first consideration after erosion protection. Certainly a vegetated bank to a seasonally or permanently wet water channel is much more attractive, as well as supporting a wide range of plants, animals and invertebrates.


“Such a sustainable measure can be considered a low carbon or even carbon neutral approach, especially when set against concrete or heavy quarried stone solutions.”


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